Does anyone know what actually breaks when a hard drive fails?

it can be alot of different things that actually fail, but the "ticking" noise you hear are the read/write heads moving across the plates of the disk.

It just means that the heads are moving across the platters trying to read/write data. Hard drives typically have a MTBF of about 50,000 hours, but of course your milage may vary :>)

Processor intensive work shouldn't have any effect on HD life, with the possible impact of the use of virtual memory (which is caching data on the HD when RAM is not available for use) The more virtual memory is used, the more read/write cycles to the HD.

My guess is that the HD is at or near the end of its useful life.

Hard drives are one of the components in computers that tend to die randomly. While shaking, heat and vibration can damage them, or make them more likely to get damaged, it isn't the only reason. Hard drive manufacturing is extremely difficult, and the drives have to keep the heads within microns of the platter without actually touching it (if they touch you'll end up with a head crash which can cause clicking sounds and a dead drive). Often times these things just happen. As with all moving parts, things fail, and don't always fail at predictable rates. Sometimes you'll get lucky and have a drive that will last for 5+ years, and sometimes they'll die quick. On the plus side, drives lifespans generally fall into one of two curves. Many drives will die in their first 6months-year of life, and many drives will start to die after 4 or 5 years of life. In between those ages drives tend to be pretty good.

Phil

Apple uses the same drives as every other computer manufacturer. You mentioned that you upgraded to more RAM. That is one way to reduce the wear and tear on a hard drive. Unfortunately, as was mentioned, hard drives are remarkably complex devices. You have a platter spinning at 5400 or 7200 RPMs (that's 90 or 120 times a second ! ). You have a read head that is floating just a few air molecules above that super fast disk. Every year, hard drive manufacturers cram more and more data storage capability into less and less space. All things considered... hard drives are remarkably reliable. Unfortunately, as with all electronic devices (mechanical or solid state)... some are going to fail. There is simply no way around that with today's production technologies. The most important thing you can do is always keep a back up of your data on some form of external media.... be it a flash drive, CD/DVD, or an external hard drive. Hard drives are fairly inexpensive... and they are easy to replace in a MacBook. What is not so easy to replace (and in the case of business use... not so inexpensive) is your data. So long as you have your data backed up, a dead hard drive becomes nothing more than a minor inconvenience.